The invention relates to the field of large industrial electrical power generators having liquid cooled stators. The invention particularly relates to liquid cooling systems for those stators.
Industrial electrical power generators are large, heavy industrial machines having internal liquid cooling systems for their stators. These stators each have a liquid cooling system referred to as a stator water cooling system (SWCS). Stator cooling fluid, e.g., oxygenated water, circulates through the SWCS to cool the windings in the stator. The cooling fluid removes heat from the stator windings generated by the high energy electrical current flowing through the windings. The SWCS includes a network of cooling passages throughout the stator and that extend between the windings. These cooling passages should remain open and free of obstructions to ensure a high flow of coolant fluid to all sections of the stator. The SWCS also includes several components external of the stator including piping, such as coolant pumps, filters, a reservoir tank and a strainer.
To ensure a continuous flow of the coolant through the SWCS, the SWCS includes a strainer to remove debris and other particles which may have become suspended within the coolant fluid. If not removed from the coolant, debris and particles tend to become clogged in and obstruct the cooling passages of the SWCS. Removal of debris and particles from the cooling fluid is needed to avoid clogging the stator cooling passages. The strainer captures debris and particles as the coolant flows through the strainer. By removing debris and particles from the cooling water, the strainer serves to keep the cooling passages open to the flow of cooling fluid.
The strainer is usually positioned in a low temperature portion of the stator water cooling system, just upstream of the generator in the coolant piping. The strainer is generally a stainless steel mesh filter located in SWCS coolant piping external and upstream of the stator. The mesh of the strainer has a tendency to accumulate copper oxide from the water of the SWCS. The oxide builds up on the wires of the mesh and reduces the openings in the mesh through which the coolant is to flow. As the deposition of copper oxide of the strainer increases, the resistance of the strainer to the coolant flow similarly increases. A strainer with oxide deposits obstructs the flow of the cooling system, reduces the flow of cool fluid through the stator, and disrupts proper cooling of the stator. There is a long-felt need for techniques that reduce the corrosion of a SWCS strainer and thereby improves the flow of coolant through the SWCS.
Deposition of copper on the SWCS strainer is often due to deposits of metals dissolved in the coolant that form on the wire mesh of the strainer. The conductive widings of the stator are generally copper (Cu). Small amounts of the cooper (Cu) from the stator windings dissolve into the coolant as the coolant flows over the windings. The dissolved copper, especially copper ions (C++), deposits on the mesh of the strainer as CuO. PCT Patent Application WO 99/43070 describes a technique for reducing cooper oxides from a stator cooling system (including the strainer) by replacing a mixed bed deionizer resin from the stator water cooling system with a larger capacity deionizer resin bed that contains only a cation resin. The cation resin chemically captures copper ions flowing in the coolant and, thus, reduces the copper ions in the coolant fluid.
Replacing the resin bed removes only copper (Cu++) ions, but does not affect other ions, e.g., CuO, that form from in the coolant. CuO forms from the copper ions Cu++ and dissolved O2 in the coolant. The increase of dissolved CuO has been found to increase the deposits that form on the strainer in the SWCS. The CuO will deposit on the mesh of the strainer if the CuO is supersaturated in the coolant. Increasing the CuO solubility level of the coolant such that the CuO is no longer supersaturated should stop further CuO deposits and dissolve existing CuO deposits. The injection of CO2 into the coolant modifies the pH of the water in the SWCS system. Increased CO2 lowers the pH of the stator water cooling system water and, thereby, increases the solubility of CuO at the generator windings. The reduction of pH increases the dissolution of CuO from the stator windings. CO2 can be injected by bubbling untreated air (which includes CO2) through the coolant.
Deposits formed on SWCS strainers are due, at least in part, to deposition of cupric oxide (CuO) on the stainless steel strainer screen. Cupric oxide deposition becomes pronounced where the stator coolant is saturated with oxygen. An analysis of cupric oxide (CuO) deposits on the strainer indicated that that the mode of deposition is by crystal growth on the strainer mesh. This mode of deposition appears to be due to an excessive concentration (supersaturated) of dissolved CuO in the coolant at the strainer. The substantial deposition by crystal growth indicates that the concentration of dissolved CuO in the SWCS coolant exceeds the solubility limit of CuO, i.e., is supersaturated. Accordingly, the CuO deposits on the mesh of the strainer appeared to be due to supersaturation of CuO in the coolant.
The CuO deposits on the SWCS strainer mesh can be substantially reduced by eliminating supersaturated levels of CuO in the coolant. The coolant will dissolve the CuO deposits if the CuO solubility level of the coolant is under-saturated. A system and technique has been developed to eliminate supersaturated levels of CuO in the SWCS coolant. To eliminate the supersaturated levels of CuO, the CuO solubility level of the coolant is increased. By increasing the amount of CuO that can be dissolved in the coolant, the amount of CuO in the coolant can remain chemically dissolved, and solid CuO deposits in the SWCS can be dissolved into the coolant. Since the CuO is fully dissolved in the coolant, the supersaturated CuO condition at the strainer is eliminated and the propensity for CuO crystal deposits on the strainer is dramatically reduced.
The solubility of CuO in the coolant is dependent on the temperature and acidity/base (pH) of the coolant. The chemical properties of CuO in a solution are well known. The solubility of CuO (as Cu++) in oxygen-saturated water is reported for different conditions of temperature and pH in D. D. MacDonald, G. R. Shierman and P. Butler, xe2x80x9cThermodynamics of Metal-Water Systems at Elevated Temperatures. Part 1. The Water and Copper-Water Systemsxe2x80x9d, AECL4136, Whiteshell Nuclear Research Establishment, Manitoba, Canada, December 1972. However, this general knowledge of CuO in solution would not have suggested that plugging of strainers in SWCS systems was due to CuO, that CuO deposits were due to crystal growth resulting from supersaturated levels of CuO in the coolant,xe2x80x9d or that the CuO depositions could be dissolved by altering the chemical and/or thermal conditions of the coolant at the strainer.
The deposits of CuO in SWCS strainers can be dissolved by adjusting the stator coolant chemistry so that the solubility of the dissolved copper ions (Cu++ and CuO is changed from a supersaturated condition to an undersaturated condition at the SWCS strainer. Increasing the solubility of CuO in the coolant causes all of the CuO in the coolant to be chemically stable and dissolves CuO deposits in the strainer (and elsewherein the SWCS), and eliminates supersaturation of CuO in the coolant. By adjusting the pH and/or temperature of the SWCS coolant water, the chemistry of the coolant water becomes strongly undersaturated in copper ions (Cu++) and CuO at the strainer.
The coolant chemistry can be adjusted, for example, by injecting CO2 into the coolant. CO2 can be injected by bubbling untreated air through the coolant. Increasing the CO2 level causes the CuO solubility level of the coolant to similarly increase. If the CuO solubility level is sufficiently increased, any CuO in the coolant remains in solution and does not deposit on the strainer. Under conditions of undersaturation, crystal growth does not occur and CuO crystal deposits on the strainer are substantially dissolved into the coolant. Without CuO crystal deposits, much of the corrosion and clogging of the mesh of a SWCS strainer is prevented. This technique of increasing the solubility of the stator coolant can be used to provide emergency clearing of the strainer, if strainer plugging is imminent.